[vsnet-grb-info 21846] GRB 190305A: Swift-XRT afterglow detection

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  23945
SUBJECT: GRB 190305A: Swift-XRT afterglow detection
DATE:    19/03/11 13:16:34 GMT
FROM:    Antonino D'Ai at IASF-PA  <antonino.dai at ifc.inaf.it>

A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAB), J.A. Kennea (PSU), B.
Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), A.P. Beardmore (U.
Leicester), J.P. Osborne (U. Leicester), P. D'Avanzo (INAF-OAB) and
P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team:

Swift-XRT has conducted further observations of the field of the AGILE
and MAXI detected burst GRB 190305A (Ursi et al. GCN Circ. 23930,
Nakahira et al. GCN Circ.23933). The observations now extend from
T0+32.7 ks to T0+143 ks. 

Of the sources reported by D'Ai et al. (GCN Circ. 23935), "Source 1" is
fading with 3-sigma significance, and is therefore likely the GRB
afterglow. Using 668 s of PC mode data and 2 UVOT images, we find an
enhanced XRT position (using the XRT-UVOT alignment and matching UVOT
field sources to the USNO-B1 catalogue): RA, Dec = 11.62657, -50.34865
which is equivalent to:

RA (J2000): 00h 46m 30.38s
Dec(J2000): -50d 20' 55.1"

with an uncertainty of 2.5 arcsec (radius, 90% confidence). This
position is 17.8 arcmin from the MAXI position. 

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.6 (+0.7, -0.6). The
best-fitting absorption column is  3.7 (+4.3, -2.9) x 10^21 cm^-2, in
excess of the Galactic value of 2.3 x 10^20 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 5.1 x 10^-11 (6.5 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.7 (+4.3, -2.9) x 10^21 cm^-2
Galactic foreground: 2.3 x 10^20 cm^-2
Excess significance: 2.0 sigma
Photon index:	     1.6 (+0.7, -0.6)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00020869.
The results of the full analysis of the tiled XRT observations are
available at http://www.swift.ac.uk/xrt_products/TILED_GRB00077.

This circular is an official product of the Swift-XRT team.